220. Evaluation of robustness to setup and range uncertainties of a proton radiosurgery plan. (December 2018) - Righetto, R.
Abstract : Purpose: To evaluate plan robustness against set up and range uncertainties on a radiosurgery plan for a small volume meningioma. Methods and materials: A single proton therapy fraction of 14GyRBE was prescribed. 3 posterior beams were planned and 3 different scenarios were generated with single field optimization (SFO): 1. uniform dose prescribed to the CTV; 2. uniform dose prescribed to the PTV; 3. uniform dose prescribed to the CTV with robust optimization assuming 2 mm as maximum patient position uncertainty and 3% as range uncertainty. Another beam arrangement (scenario 4) including one posterior and 2 lateral field was planned using the robust optimization options of scenario 3. Each nominal plan was recomputed moving the isocenter on vertices of a cube of side length 3 mm, centered at the planning isocenter and applying a density perturbation of +-3%. The DVH for the CTV and for the brain tissue were collected. The volume of CTV receiving a dose greater than 1400cGyRBE (V1400) and the dose to 5 cc of brain tissue were compared for the nominal plan and the worst case scenario. Results: The DVH for CTV and brain tissue are shown in Fig. 1. V1400 and the dose to 5 cc of brain tissue are presented in Tab. 1. V1400 of the CTV is between 96 and 99% for all nominal plans, in the worst case V1400 drops to 54% for scenario 1 and remains greater than 80% for the other scenarios where a margin was used in planning. In scenario n.4 beams arrangement contributes to the robustness of V1400. Conclusion: The use of appropriate margins and independent beam directions contribute to plan robustness and robustness analysis may be a part of proton therapy planning even in case of SFO for treatments delivered in one session where a set-up error is systematic.